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Astron. Astrophys. 341, 818-826 (1999)

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Structural properties of s-Cepheid velocity curves *

Constraining the location of the [FORMULA] resonance

F. Kienzle 1, P. Moskalik 2, D. Bersier 3 and F. Pont 1

1 Observatoire de Geneve, Ch. des Maillettes 51, CH-1290 Sauverny, Switzerland
2 Copernicus Astronomical Center, ul. Bartycka 18, PL-00-716 Warsaw, Poland
3 Mt. Stromlo and Siding Spring Observatories, Weston Creek, ACT 2611, Australia

Received 21 July 1998 / Accepted 21 October 1998


The light curves of the first overtone Pop. I Cepheids (s-Cepheids) show a discontinuity in their [FORMULA] vs. [FORMULA] diagram, near [FORMULA] day. This feature, commonly attributed to the 2:1 resonance between the first and the fourth overtones ([FORMULA]), is not reproduced by the hydrodynamical models. With the goal of reexamining the resonance hypothesis, we have obtained new CORAVEL radial velocity curves for 14 overtone Cepheids. Together with 10 objects of Krzyt et al. (1999), the combined sample covers the whole range of overtone Cepheid periods. The velocity Fourier parameters display a strong characteristic resonant behavior. In striking contrast to photometric ones, they vary smoothly with the pulsation period and show no jump at 3.2 day. The existing radiative hydrodynamical models match the velocity parameters very well. The center of the [FORMULA] resonance is estimated to occur at [FORMULA] day, i.e. at a period considerably longer than previously assumed (3.2 day). We identify two new members of the s-Cepheid group: MY Pup and V440 Per.

Key words: techniques: radial velocities – stars: oscillations – stars: variables: Cepheids

* Based on observations collected at the European Southern Observatory (La Silla, Chile) and at the Observatoire de Haute-Provence (France)

Send offprint requests to: F. Kienzle

Correspondence to: Francesco.Kienzle@obs.unige.ch

© European Southern Observatory (ESO) 1999

Online publication: December 16, 1998